1. Field of the Invention
The present invention refers to a procedure and an apparatus for the survey of the impression made on a specimen in measuring the hardness at penetration, and in particular, for the determination of the position of characteristic points of such impression, for example, the vertices of the Vickers impression or the ends of two orthogonal diameters of the Brinell impression.
The invention refers in particular to a procedure of the type in which the specimen is illuminated in such a way as to produce, in correspondence of the impression, a luminance variation respect to the surrounding zone, and in which the impression image is then magnified by an optical system.
2. Summary of the Prior Art
As everybody knows, the hardness at penetration is given, substantially, by the ratio between the applied load and the surface area of the impression produced on a specimen by a indentor. The indentor of the Vickers hardness test is a square-based pyramidal diamond; in the Brinell hardness test it is a ball made of steel or of tungsten carbide. According to the known technique, the characteristic measures (diagonals, diameters, etc.) of the impression are determined by means of a good microscope, with magnifications of 100-650 diameters, equipped with a micrometer eyepiece under the axis of which the specimen with the impression is brought.
The surface area of the impression is then calculated on the base of the means value of the characteristic measures thus obtained.
The hardness measurement so carried out results hardly repeatable and, what is more serious for practical purposes, is affected by a remarkable uncertainty; not only the hardness values measured on the same specimen by using different test machines may result different between them, but also measurements performed by two operators using the same test machine may result different even to a remarkable degree. The main limit, inherent in the method of measurement described above is due to the subjective error by the operator, in particular when measuring the lengths or the characteristic dimensions of the impression.
Further limits of the conventional method of hardness measurement at penetration stem from the laboriousness and long time required by the measurement itself. The stress resulting by long lasting observations performed when using hardometers equipped with microscopes (usually with a single eyepiece) tends to reduce the operator's concentration capacity and, therefore, to increase the inaccuracy and uncertainty of the measurement.
Methods and procedures have been proposed with a view of reducing the incidence of the subjective error by the operator, for example by taking the mean value of the measurements performed by two operators. Such methods, far from having removed the problem of the subjective factor in determining the hardness value, though allowing for a modest reduction of the measurement uncertainty, require long measurement times and thus much greater costs.
It has been found that the magnitude of discrepancies between measurements carried out by several operators is of the same order as the optical system resolution. As a consequence, it has been suggested to use optical instruments having a better resolution. However, investigations carried out to this purpose on images of impressions viewed through scanning electronic microscopes have shown that the impression edges are not well defined at all. It could be stated then, that the above mentioned subjective error committed by the operator might be probably due to a different and subjective way of locating the characteristic points of the impression which actually have a locally undefined configuration.